I think it can take a few weeks or more for some of them - sometimes the quieter ones - to find friends, feet, and so on. So the message at this stage (to dparents and dstudents!) is don't panic if you're not enjoying it yet. But it can be hard to think of your dc in that position!
It's difficult for them - the impression people get is that at university everyone immediately makes wonderful friends and has a great time in the first week, whereas if you think about it that's not usually how friends are made at work and in life - it's a gradual process.
I agree encourage club/society joining, including things they've never done at home - there are sometimes sports clubs for beginners, which might be promising? Everyone learning something new together can be quite a good way of socialising.